Hydrofoils?

Discussion in 'Boat Design' started by Nick F, Sep 19, 2008.

  1. Nick F
    Joined: Aug 2008
    Posts: 19
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    Location: Auckland

    Nick F Junior Member

    Hay guys

    I are currently working on a hydrofoil design/ research program to develop some different foil configurations for powered craft. one of our designs is very sensitive to the pitching moment due to the pressure distribution over a cambered foil section(CmAC).
    My question is this. dose a delta shaped foil (similar to sky chair) have a pitching force the acts in the same way as a straight blade section?
    Also can the pitching moment on either foil section be assumed to act through any one point on the foil? and how much dose this point move on relation to alpha?

    We keep getting very in consist results

    All of our data is collected through real size testing with a special rig mounted on the stern of a test boat. And the foil is mounted 400mm below the hull.

    Nick F
    :)
     
  2. Guest625101138

    Guest625101138 Previous Member

    I can give you a partial answer.

    There will only be one resultant force acting on a foil. By convention this force is resolved into lift, drag and moment components. The drag is in line with the flow. The lift is perpendicular to drag and taken as acting through the aerodynamic centre, usually taken as 25% of chord. The moment is the pitching moment required to stabilise the foil with the lift assumed to act at 25% of chord. So only ONE force but for ease of analysis resolved into three components.

    The moment usually does change with AoA. All it means is the pressure distribution changes as the AoA changes.

    You can do quite easy analysis of foils using JavaFoil. Takes a bit of understanding but might save a lot of trial and error.
    http://www.mh-aerotools.de/airfoils/jf_applet.htm
    You must set the parameters in the options for water.

    I believe a delta wing is more tolerant to stall than a straight wing. By this I mean the stall is more progressive. So part of the wing can be in stall conditions while other section is still providing good lift to drag. There are sites that you can Google that will provide discussion on this. The only difference between a foil in air and a foil in water is that the one in water will suffer extra drag due to wave generation as it approaches the surface.

    The foils on the sky ski have quite poor lift to drag. They will suffer high induced drag because they are low aspect foils. JavaFoil has correction for foil aspect so you can determine this factor for a given foil and operating conditions.

    Rick
     
  3. Nick F
    Joined: Aug 2008
    Posts: 19
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    Location: Auckland

    Nick F Junior Member

    Thanks Rick

    As you have said. the forces are presumed to act through the 25% cord. But our foil is very sensitive to the forces (especially the pitching moment)
    My main question is dose the forces on a delta shaped foil act through any one point. As on a delta shaped foil the cord is varying as the foil shape changes . so 25% of what cord?

    I have attached two pic to illustrate how the foils are attached and how they pivot(pic # 1,2. (similar to design of Kotaro Horiuchi))
    If both of these foil shape have a large pitching moment the forces acting on the surface control wand will be extremely high and increasing with speed. But the amount of force transferred to the control wand will be effected buy the position of the pivot point on the foil. This is were it gets hazy for me.

    The foil chair has a tail on it.(pic #3) Is this tail designed to stabilize the main foil? As the pitching moment is increased with the square of the speed so the will be the lift on the tail? Correct? So in effect we have a stable configuration at all speeds?:confused:
    Nick
     

    Attached Files:

  4. Guest625101138

    Guest625101138 Previous Member

    I looked at foil interaction where there are two foils and there is nothing apparent in this regard.

    There would be some self-stabilising in near surface operation. When the tail is closer to the surface it will lose lift. Typically this becomes significant within 3 chord lengths of the surface. So if the tail is near the surface it will lose lift and drop somewhat thereby increasing the angle of attack of the both foils and they will nose up a little. When the nose is near the surface it will lose lift and put the foils into a dive. So the centre of lift can move quite a lot depending on proximity to the surface. It would increase response time for pitch control through weight distribution or other correcting moments from towing rope for example.

    A single foil will not have the same scope to shift the centre of lift so will be much more twitchy in response. It should react faster but will therefore be harder to control using human input.

    As far as I can determine the only stabilising control is related to surface proximity. So separation of the two foils would increase the ability to shift centre of combined lift and slow response time.

    The delta shape on the front foil is likely based on trial and error to give good banking in a turn without aerating and able to operate at high angle of attack without stalling during lift off. (I have not seen these things in operation so am only speculating based on how I expect them to operate.)

    Rick
     
  5. Guest625101138

    Guest625101138 Previous Member

    I arranged the foils a little closer and there are slight stabilising influences. At negative angle of attack the lift on the tail plane changes more rapidly than the lift on the main foil. The attached shows the flow field at zero angle of attack. You can see there is some negative pressure at the front of the tail foil due to the change in air flow over the front foil.

    So this would help stabilise as well but the surface proximity will be more significant in normal operation I expect.

    The thing is a bit like ballancing a broom stick on the hand. The longer the broom and heavy the brush the easier it is to keep vertical because it has a slow response time.

    Rick
     

    Attached Files:


  6. Guest625101138

    Guest625101138 Previous Member

    Looked at some video clips to see how they shift weight to control pitch. The really good guys make gross shifts in body weight to alter the angle of attack. The lift forces are quite significant as can be seen in this one:
    http://www.youtube.com/watch?v=4mf0nwvYmBE&feature=related

    Less experienced riders get unstable with the board rising up and down. If the nose of the board digs in they lose it.

    Rick
     
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